When manufacturing/processing a spectacle lens, it is necessary to evaluate the obtained spectacle lens to see whether or not the both optical surfaces thereof have the required optical performance and surface shape set forth in the applicable specification or design. The spectacle lens evaluation is mainly performed by measuring the optical characteristics at measurement point(s) of the lens. Generally, if the spectacle lens is a single-vision lens, then an optical center is selected as the measurement point; if the spectacle lens is a multi-focal lens, then a distance portion optical center and positions for measuring addition power (i.e., a distance portion vertex power measurement point and a near portion reference point) are selected as the measurement points; and if the spectacle lens is a progressive-addition lens, then a distance portion reference point and positions for measuring addition power (i.e., a distance portion reference point and a near portion design reference point) are selected as the measurement points.
Incidentally, measuring methods for measuring the optical characteristics of various kinds of spectacle lenses (the single-vision spectacle lens, the multi-focal spectacle lens and the progressive-addition lens) and allowance for measured value are specified in ISO (International Organization for Standardization), JIS (Japanese Industrial Standards) and the like.
However, since the wearer of the spectacle lens also sees things through the region other than the measurement point(s) of the lens, there is a desire to develop an evaluation method in which the lens is evaluated in a wide region, instead of being evaluated at the aforesaid measurement point(s) only. For example, it is important to evaluate the lens in a wide region particularly in the case where one or both surfaces of the lens have complicated surface shape, such as a progressive-addition lens.
In most conventional progressive-addition lens, a surface on the object side (a front surface of the lens) is formed as a “progressive-power surface”, and a surface on the eyeball side (a back surface of the lens) is formed as a “spherical surface” or an “astigmatic surface”. However, recently there is developed a bi-aspherical type progressive-addition lens whose both surfaces (the front surface and the back surface of the lens) are formed as aspheric surfaces, and the progressive-power is obtained by combining the both surfaces (see, for example, Japanese Patent Registration No. 3617004 Feb. 2, 2005).
Further, as prior arts, it is proposed that the lens is evaluated by actually measuring a three-dimensional shape of the lens surface, and the optical characteristics are calculated based on the three-dimensional shape (see, for example, Japanese Patent Registration No. 3617004 and Published Japanese Translation of PCT International Publication for Patent Application Publication No. H10-507825 Jul. 28, 1998). Further, it is also proposed to provide a method and device to measure the optical characteristics, such as a dioptric power distribution (referred to as a “power distribution” hereinafter), of the lens in a wide region (see Japanese Unexamined Patent Application Publication No. H08-304228 Nov. 22, 1996), and evaluate the optical characteristics of the lens based on a difference distribution between a measured power distribution in a wide region and a power distribution obtained based on design data (see Japanese Unexamined Patent Application Publication No. 2000-186978 Jul. 4, 2000).
Further, since a mold used for molding the spectacle plastic lens is formed of glass whose molding surface is transferred to the plastic lens, the molding surface of the mold has to be formed with the same accuracy as that of the surface of the spectacle lens. Thus, in the case where the mold is formed to be conformed to the shape of the lens, the mold can be evaluated in the same manner as the spectacle lens.
However, in the bi-aspherical type progressive-addition lens disclosed in Japanese Patent Registration No. 3617004, not only the shape of the both surfaces (the front surface and the back surface) needs to be finished according to pertinent design, but also the front surface and the back surface need to be combined correctly in position. If a relative positional shift (including positional shift in up/down/left/right directions, rotational shift, and the combination thereof) is generated between the front surface and the back surface, the influence thereof may be exerted on the optical performance depending on the degree of the positional shift.
An object of the present invention is to provide a method for easily evaluating whether or not the relative positional shift generated between the both surfaces is within an allowable range during the manufacturing process of the bi-aspherical type lens.